An ultrasonic transducer is an electronic device used to emit and receive sound waves at frequencies beyond the human detection limit. Ultrasonic transducers are used in medical imaging, non-destructive evaluation, and robotic sensing among other uses. The most common form of ultrasonic transducers are piezoelectric transducers. Piezoelectric transducers are not efficient in the conversion between electric and acoustic energy in air. The operating frequencies of piezoelectric transducers in air are quite low. Magneto-strictive and capacitive transducers have also been used. These transducers operate in the low MHz range and are narrow band devices.
In co-pending application Ser. No. 08/327,210, filed Oct. 21, 1994, there is a described a narrow band microfabricated ultrasonic transducer. The transducer consists of circular silicon nitride membranes suspended above a heavily doped silicon substrate. FIGS. 1 and 2 schematically illustrate the microfabricated multi-element transducer described in said co-pending application. The transducers include a plurality of identical individual membranes 11 suspended above a silicon substrate 12 by silicon dioxide 13.
Microfabricated ultrasonic transducers efficiently excite and detect airborne ultrasonic waves in that they use thin resilient resonant membranes with very little inertia. The momentum carried by approximately half of a wavelength of air molecules is thus able to set the membrane in motion and visa versa. Electrostatic actuation and detection enable the realization and control of such resonant membranes. When distances are small, electrostatic attractions can exert very large forces on the actuators of interest. Because the membranes forming the multi-element microfabricated ultrasonic transducer described in the copending application are all of essentially the same size, the transducer is inherently a narrow band device as shown by the curve A in FIG. 7.
One of the most important figures of merit of an ultrasonic transducer is the range of frequencies over which it can operate. This range is referred to as the transducer's bandwidth. From Fourier theory, it can be shown that bandwidth is inversely proportional to the time resolution of a device. That is, the broader a transducer's bandwidth, the narrower a time interval it can measure over. For example, a bell is a narrow band device. When it is struck by an object (an event lasting only a very short time) it rings for a relatively long time. Consequently, it is very difficult for a listener, just by hearing the sound of the bell, to determine exactly when the bell was struck. A broader band device, such as a stick, does not ring much and thus allows for finer time discrepancy.
The vast majority of ultrasonic applications consist of mapping the time delay of echoes to spatial coordinates. In short, the precision and resolution of the time measurement capabilities of an ultrasonic system are directly related to the spatial precision and resolution of the system. It is thus clear that for ultrasonic systems ranging from simple position detectors, to gas flow meters, to complex medical imaging equipment, the availability of ultra-broadband devices would constitute a major advance. A secondary advantage to broadband transducers is that even in narrow band applications such as ultrasonic resonance experiments, the same transducers can be used over the frequencies of interest. Currently, a separate transducer is needed to measure at each different frequency of interest.